The invention pertains to fuses which may be fabricated as part of an integrated circuit substrate. Such fuses are useful in such applications as microwave diode power amplifiers wherein a plurality of diodes are connected in parallel in each amplifier. In such a connection, if one of the diodes fails by short-circuiting and there is no protection, the entire amplifier also fails and becomes inoperative. By inserting a fuse in series with the DC bias connection to each of these diodes, a single diode failure will not result in catastrophic failure of the entire amplifier. In most applications, the overall unit, of which the amplifier is a part, can continue to function to a large degree with a single partially operating amplifier.
Another application where such fuses are used to advantage is where a number of power transistors in an integrated circuit in individual units are operated in parallel. In such a case, a fuse may be inserted into the emitter leg of each transistor on the semiconductor chip so that a short circuit failure of a single transistor will blow the fuse thereby removing the transistor from the circuit rather than causing the entire circuit to fail. The fuse can also serve as the emitter ballast resistor for each output power transistor by including resistive material in the fuse. In these and other applications, it is important that the current at which the fuse blows be predictable and controllable to a great degree for proper protection of the device to which the fuse is connected. With prior art integrated circuit fusing techniques, the fuse was fabricated directly upon the semiconductor substrate. Since different regions of the underlying semiconductor substrate conduct heat to different degrees depending upon the dopant level in the semiconductor material, heat flowing directly from the fusible link into the substrate caused serious problems in the determination of the current at which the fuse would blow. Problems were also caused in that adjacent devices could be damaged by the heat flowing from the fuse into the substrate.
Many types of read only memories also employ integrated circuit type fuses. For example, in one such memory, in order to program a zero into a given memory location, it is necessary to blow a fuse at the corresponding location in the diode storage matrix on the read only memory semiconductor chip. It is especially important in that application that the fuses blow at a pre-specified current; otherwise the entire unit will be prone to failure. If the fuse were to blow at too low a current, the fuse could blow during normal device operation resulting in a change of stored data. If the fuse were to require too large current to blow it, it would not be possible to program the correct data into the memory as a limited amount of current is typically available from driving devices used to blow the fuse.